This work focuses on low-purity kaolin, widely accessible throughout the globe. Room temperature cured geopolymer mortars (GPMs) were formulated using an aluminosilicate precursor based on calcined lithomarge and potassium silicate activator. The effect of mix proportion parameters on the engineering properties of GPMs was investigated. The behaviour of GPMs was compared with that of Portland cement-based mortars (PCMs). Statistically designed experiments revealed that an increase in water-to-solid (w/s) ratio had a dominant effect on increasing the workability and setting time while decreasing the compressive strength of GPMs. In contrast to PCMs, GPMs proportioned with a constant water content showed a non-linear relationship between the w/s ratio and workability, which could be associated with changes to paste/sand proportions and/or water/alkali proportions. Like-for-like comparison of GPMs and PCMs showed that GPMs require lower free water content, and can offer shorter setting times and a rapid strength development.